The capacity of mobile communications systems is typically limited by the amount of available radio resources. The radio resources can be defined as units in a resource space spanned by e.g. time, frequency and/or codes. The total capacity of a communications system may be increased by letting geographically well separated nodes use the same radio resources, a so-called reuse of resources. However, in order to avoid interference between nodes using the same radio resources, the geographical distance (or distance defined by radio conditions) between the nodes has to be relatively large. Any co-channel interference between the two users of the same radio resource should not substantially degrade quality and performance in either of the cells. In the present disclosure, the term “co-channel interference” refers to interference caused by nodes utilising the same radio resources, in a general case determined in terms of e.g. frequency, time and/or code. However, smaller amounts of co-channel interference may be tolerated depending on the amount of sophistication of signal processing at the receivers, e.g. in terms of coding, diversity, interleaving etc. The more co-channel interference receivers can tolerate, the more system operators can decrease the reuse distance and thereby increase the total capacity.
A radio signal is distorted when it is transmitted over the air. The received signal therefore differs somewhat in amplitude and/or phase compared with the originally transmitted one. This is basically due to path loss due to the traveled distance, shadowing effects and multipath fading. In order to correctly decode the transmitted data, model parameters of the signal distortion are estimated. One approach that often is used is to incorporate a known symbol sequence into the transmitted signal. Such known symbol sequence is known under different names, such as “training sequence”, “learning sequence”, “training bits”, “pilot sequence” etc. In the present disclosure, the term “training sequence” will be used for characterising all types of transmitted data, the content of which is pre-known by the receiver, and which is used to identify and characterise the received signals, e.g. for synchronisation and channel estimation.
The training sequence has typically a double purpose. One purpose is to provide means for achieving reliable channel estimations. Another purpose is to provide means for the receiver to distinguish between co-channel signals originated from different transmitters. The different training sequences have preferably a low cross-correlation, i.e. the sum of products of corresponding symbols of two training sequences should be close to zero at all relative time shifts. At the same time, the auto-correlation should be such that a high correlation is achieved for a zero bit shift, while a low correlation is achieved for any other bit shifts. This means that the number of useful training sequences is severely limited and a set of training sequences with good cross-correlation and auto-correlation properties is often difficult if not impossible to find. In GSM, a 26 bits long training sequence is used in a normal burst. Totally 8 different training sequences are available within the GSM specifications. All of these training sequences have very good auto-correlation properties, but some of them have unfortunately relatively high cross-correlation among each other.
If two signals are transmitted using the same radio resource, but with different training sequences, a receiver can use the training sequence to distinguish between the signals during the estimation procedures. The influence from an interfering co-channel signal having a different training sequence can thus be fairly well mitigated. However, if both signals also use the same training sequence, the receiver will interpret the total signal as coming from one and the same transmitter. This will result in erroneous channel estimation, inadequate path loss determinations, incorrect directional determinations etc. In the present disclosure, we refer to this kind of co-channel interfering signals, which use training sequences that have substantially high cross-correlations with the training sequence used by the desired signal, as “co-sequence interference”. The term “co-sequence interference” thus in a general case comprises not only interference between signals utilising identical training sequences, but also interference between signals having training sequences that significantly influences the interpretation of the desired signal due to high cross-correlation.
In the published international patent application WO 98/59443 it is concluded that if two signals arrive at a receiver at almost the same time, and their training sequences are the same, there is, in conventional receivers, no way to distinguish the contribution from each of them to the received signal. Instead of mitigating the effects of such interference, the disclosure presents a method for preventing or reducing the risk of the interference to appear by introducing time offsets between radio base stations using the same resources. Similar preventing ideas are also disclosed in the published international patent application WO 98/59443. A problem with such a solution is that it becomes more difficult to provide efficient radio network planning and to achieve maximum gain from e.g. interference suppression techniques if bursts within a system are not time-aligned.
Also in the published U.S. patent application US 2003/0026223, the existence of the co-sequence interference is discussed. In this disclosure, the interference is not directly detected or compensated for. Instead, the effects of any possible co-sequence interference are reduced by introducing training sequence hopping for all connections, whereby the continuous time interval during which any possible co-sequence interference exists is reduced.